We have sought to understand the mechanisms by which murine leukemia viruses induce tumors by identifying 1) preleukemic changes in the lymphocyte compartment of MuLV-infected mice using fluorescent reagents and flow cytometry; 2) specific defects in lymphocyte ontogeny and function that confer resistance to or alter the target for transformation using inbred mouse strains; and 3) the region of the MuLV genome responsible for transforming specific lymphoid cells by generating recombinants between MuLVs with different targets.